Creating smooth color regions with panel.contourplot()

When I use panel.contourplot() with filled color regions, the coloring
 follows the stair-step edge of the underlying grid instead the smooth
 contour lines themselves.  How can I get the latter behavior?  I would
 guess there is a much simpler way than manually creating polygons with
 contourLines(), especially since a contour interval/region can have
 holes inside it (different contour intervals).

Manually creating polygons with contourLines will not really help
because (1) as you noted, there will be holes, and (2) contours that
cross edges will be open. The only real solution is to color each
rectangle individually, and that would be very inefficient in R code
(grid does not have a C-level interface).

The good news is that filled.contour() already does this efficiently,
and you can use that through the gridBase package:


panel.filledcontour <-
    function(x, y, z, subscripts,
             at,
             col.regions = cm.colors,
             col = col.regions(length(at) - 1),
             ...)
{
    stopifnot(require("gridBase"))
    z <- matrix(z[subscripts],
                nrow = length(unique(x[subscripts])),
                ncol = length(unique(y[subscripts])))
    if (!is.double(z)) storage.mode(z) <- "double"
    opar <- par(no.readonly = TRUE)
    on.exit(par(opar))
    if (panel.number() > 1) par(new = TRUE)
    par(fig = gridFIG(), omi = c(0, 0, 0, 0), mai = c(0, 0, 0, 0))
    cpl <- current.panel.limits()
    plot.window(xlim = cpl$xlim, ylim = cpl$ylim,
                log = "", xaxs = "i", yaxs = "i")
    .Internal(filledcontour(as.double(do.breaks(cpl$xlim, nrow(z) - 1)),
                            as.double(do.breaks(cpl$ylim, ncol(z) - 1)),
                            z, as.double(at), col = col))
}

plot.new()

levelplot(volcano, panel = panel.filledcontour,
          col.regions = terrain.colors,
          cuts = 25)

-Deepayan